The light-emitting diode (LED) is a semiconductor solid-state light-emitting device which results from joining a p-type semiconductor and an n-type semiconductor. The LED is used as the light source for various devices because it possesses strong points, such as long lifetime, excellent resistance to shock, low power consumption and high reliability and permits reduction in volume, thickness and weight. Particularly, the white LED is used for disaster prevention illuminations requiring reliability, on-vehicle illuminations and liquid crystal backlights favoring reduction in volume and weight, and railroad destination guide plates necessitating visibility and is expected to find application to general household room illuminations.
When an electric current is made to flow in the forward direction of the p-n junction formed of a direct transition type semiconductor, the electrons and the positive holes are recombined and the light possessing the peak wavelength corresponding to the forbidden band width of the semiconductor is consequently emitted. Since the emission spectrum of the LED generally has a narrow half width of the peak wavelength, the color of the emission of the white LED is exclusively obtained by the principle concerning the mixture of the colors of the light.
As means to obtain the white color, specifically the following methods have been known.
(1) A method which comprises combining three kinds of LED's individually emitting a red color (R), a green color (G) and a blue color (B), which are the three primary colors of light, and mixing these LED lights,
(2) A method which comprises combining an ultraviolet LED emitting an ultraviolet light and three kinds of fluorescent materials individually excited by the ultraviolet light to emit a red color (R), a green color (G) and a blue color (B) respectively and mixing the fluorescences of the three colors emitted from the fluorescence materials, and
(3) A method which comprises combining a blue LED emitting a blue color and a fluorescent material excited by the blue color light of the blue LED to emit the fluorescent lights of a blue color and a yellow color which is in the relationship of a complementary color and mixing the LED light in the blue color with the yellow light emitted from the fluorescent material.
The method which obtains a prescribed emission color by using a plurality of LEDs requires a special circuit for adjusting the electric currents of the individual LEDs for the purpose of balancing the component colors. In contrast, the method which obtains a prescribed emission color by combining an LED and a fluorescent material is at an advantage in obviating the necessity for such a circuit and lowering the cost of LED. Thus, various proposals have heretofore been made regarding the fluorescent materials of the kind using an LED as a light source.
For example, a YAG fluorescent material resulting from doping with Ce a YAG-based oxide host crystal represented by the compositional formula (Y, Gd)3(Al, Ga)5O12 has been disclosed (refer to Takashi Mukai et al, Applied Physics, Vol. 68, No. 2 (1999), pp. 152-155). This literature contains a description to the effect that by coating the first surface of an InGaN-based blue LED chip with a thin layer of a YAG fluorescent material, it is made possible to induce a mixture of a blue light emitted from the blue LED and a fluorescence of a peak wavelength of 550 nm emitted from the YAG fluorescent material owing to the excitation by the blue light and consequently obtain a white light.
A white LED resulting from combining a light-emitting device, such as a nitride-based compound semiconductor capable of emitting an ultraviolet light, and a fluorescent material made to emit light owing to the excitation by the ultraviolet light is also disclosed. As fluorescent materials usable herein, a composition (Sr, Ca, Ba)10(PO4)6Cl2:Eu emitting a blue light, a composition 3(Ba, Mg, Mn)0·8Al2O3:Eu emitting a green light and a composition Y2O2S: Eu emitting a red light are disclosed (refer to JP-A 2002-203991).
The existing fluorescent materials generally are at a disadvantage in suffering the spectral intensity to be markedly decreased when the excitation wavelength exceeds the near ultraviolet region.
The white LED obtained by coating the first surface of the InGaN-based blue LED chip with a fluorescent material formed of a YAG-based oxide has been reputed to acquire high luminance with difficulty because the excitation energy of the YAG-based oxide as the fluorescent material and the excitation energy of the blue LED as the light source are not in accord with each other and because the excitation energies are not transformed satisfactorily efficiently.
Further, a white LED formed by combining a light-emitting device, such as a nitride-based compound semiconductor which is capable of emitting an ultraviolet light, and a fluorescent material excited by the ultraviolet light to emit light has been reputed to acquire high luminance with difficulty because it entails the problem that the mixing ratio of the fluorescent material of the red color component is unduly increased because the light-emitting efficiency thereof is appreciably lower than that of the other fluorescent material.
This invention is aimed at providing an oxynitride-based fluorescent material which enables a white light-emitting diode (white LED) using a blue light-emitting diode (blue LED) or an ultraviolet light-emitting diode (ultraviolet LED) as a light source to acquire enhanced luminance and a light-emitting device using the fluorescent material.
The present inventor has pursued a diligent study with a view to accomplishing the above object and has consequently discovered that an oxynitride-based fluorescent material formed by substituting Eu for part of M in the composition represented by the general formula MOSi2N2O (provided that M denotes one or more elements selected from among Be, Mg, Ca, Sr and Ba) possesses a strong absorption band in a wide range extending over ultraviolet to near ultraviolet to visible light and has discovered anew that a white LED using the fluorescent material of this invention possesses excellent emission properties. This invention has been perfected as a result.